A typical terrestrial signal receiver includes a wide-band radio frequency (RF) stage in which broadcast signals are amplified. These signals are then converted to an intermediate frequency (IF) by a mixer, filtered and further amplified by an IF amplifier stage to provide a narrow-band signal to a demodulator.
Terrestrial receivers typically include one or more variable gain amplifier (VGA) stages to accommodate the wide range of received signal power that may be encountered in practice. The gains of these amplifiers are adjusted to achieve a desired signal level at the demodulator. For example, one VGA circuit may amplify the broadcast signals at the RF stage and another may amplify the signals at the IF stage. It is the function of the automatic gain control (AGC) algorithm to set the gain of each VGA such that the power of the desired signal presented to demodulator is maintained at approximately the prescribed level. Furthermore, because amplifiers can introduce spurious electrical signals (i.e. “noise”) into the signal and because non-linearities in amplifiers and mixers can distort the amplified signal, practical aspects of AGC circuits including device “noise figure” and “linearity” are desirably considered when adjusting the gain of each VGA.